SyncItemPostAction.cs

[Serializable]
public class SyncItemPostAction : Command
{
    public override void Execute(CommandContext context)
    {
        var contextItem = context.Items[0];
        if (contextItem == null || contextItem.TemplateID != CommonConst.TemplateIdToCheck)
        {
            return;
        }

        var newFieldValue = contextItem["Custom Field"];
        // custom code to use the new field values
    }
}